Those skilled in the arts of antennas and arrays know that antennas are transducers which transduce electromagnetic energy between unguided- and guided-wave forms. More particularly, the unguided form of electromagnetic energy is that propagating in “free space,” while guided electromagnetic energy follows a defined path established by a “transmission line” of some sort. Transmission lines include coaxial cables, rectangular and circular conductive waveguides, dielectric paths, and the like. Antennas are totally reciprocal devices, which have the same beam characteristics in both transmission and reception modes. For historic reasons, the guided-wave port of an antenna is termed a “feed” port, regardless of whether the antenna operates in transmission or reception. The beam characteristics of an antenna are established, in part, by the size of the radiating portions of the antenna relative to the wavelength. Small antennas make for broad or nondirective beams, and large antennas make for small, narrow or directive beams.
When more directivity (narrower beamwidth) is desired than can be achieved from a single antenna, several antennas may be grouped together into an “array” and fed together in a phase-controlled manner, to generate the beam characteristics of an antenna larger than that of any single antenna element. The structures which control the apportionment of power to (or from) the antenna elements are termed “beamformers,” and a beamformer includes a beam port and a plurality of element ports. In a transmit mode, the signal to be transmitted is applied to the beam port and is distributed by the beamformer to the various element ports. In the receive mode, the unguided electromagnetic signals received by the antenna elements and coupled in guided form to the element ports are combined to produce a beam signal at the beam port of the beamformer. A salient advantage of sophisticated beamformers is that they may include a plurality of beam ports, each of which distributes the electromagnetic energy in such a fashion that different antenna beams may be generated simultaneously. The advantages of antenna arrays over single-antenna transducers has led to extensive use of array antennas. A notable disadvantage of antenna arrays is that, when the antenna beam is scanned away from broadside to the array, the polarization response of the elemental antennas (elements) of the array is degraded, and the larger the scan angle, the greater the degradation, because the element-level phase progression that steers the beam results in tilting of the beam relative to each element. Reflector-type antennas, however, are mechanically steered to point the beam, and provide high cross-polarization purity. The degraded off-axis cross-polarization performance of antenna arrays has inhibited the use of such arrays for polarimetric weather measurements and research.
Improved radar using array antennas for polarimetric measurements is desired.